This invention relates to tunable radio frequency devices, and more particularly, to voltage-tuned dielectric varactors.
Varactors are voltage tunable capacitors in which the capacitance can be changed by applying an electric field to the device. This property has wide applications in electrically tuned radio frequency circuits, such as tunable filters, phase shifters, delay lines, voltage controlled oscillators, etc. The most commonly used varactor is a semiconductor diode varactor, which generally has a low quality factor, Q, especially at high frequencies, low power handling capacity, low third intermodulation product (IP3), and a limited capacitance range. Another type of voltage tunable varactor uses ferroelectric materials.
Tunable ferroelectric materials are materials whose permittivity (more commonly called dielectric constant) can be varied by varying the strength of an electric field to which the materials are subjected. Even though these materials work in their paraelectric phase above the Curie temperature, they are conveniently called xe2x80x9cferroelectricxe2x80x9d because they exhibit spontaneous polarization at temperatures below the Curie temperature. Tunable ferroelectric materials including barium-strontium titanate (BST) or BST composites have been the subject of several patents.
Dielectric materials including barium strontium titanate are disclosed in U.S. Pat. No. 5,312,790 to Sengupta, et al. entitled xe2x80x9cCeramic Ferroelectric Materialxe2x80x9d; U.S. Pat. No. 5,427,988 to Sengupta, et al. entitled xe2x80x9cCeramic Ferroelectric Composite Material-BSTOxe2x80x94MgOxe2x80x9d; U.S. Pat. No. 5,486,491 to Sengupta, et al. entitled xe2x80x9cCeramic Ferroelectric Composite Materialxe2x80x94BSTOxe2x80x94ZrO2xe2x80x9d; U.S. Pat. No. 5,635,434 to Sengupta, et al. entitled xe2x80x9cCeramic Ferroelectric Composite Material-BSTO-Magnesium Based Compoundxe2x80x9d; U.S. Pat. No. 5,830,591 to Sengupta, et al. entitled xe2x80x9cMultilayered Ferroelectric Composite Waveguidesxe2x80x9d; U.S. Pat. No. 5,846,893 to Sengupta, et al. entitled xe2x80x9cThin Film Ferroelectric Composites and Method of Makingxe2x80x9d; U.S. Pat. No. 5,766,697 to Sengupta, et al. entitled xe2x80x9cMethod of Making Thin Film Compositesxe2x80x9d; U.S. Pat. No. 5,693,429 to Sengupta, et al. entitled xe2x80x9cElectronically Graded Multilayer Ferroelectric Compositesxe2x80x9d; and U.S. Pat. No. 5,635,433 to Sengupta, entitled xe2x80x9cCeramic Ferroelectric Composite Material-BSTOxe2x80x94ZnOxe2x80x9d. These patents are hereby incorporated by reference. In addition, copending, commonly assigned United States patent applications Ser. No. 09/594,837, filed Jun. 15, 2000, and Ser. No. 09/768,690, filed Jan. 24, 2001, disclose additional tunable dielectric materials and are also incorporated by reference. The materials shown in these patents, especially BSTOxe2x80x94MgO composites, show low dielectric loss and high tunability. Tunability is defined as the fractional change in the dielectric constant with applied voltage.
One tunable dielectric varactor is shown in U.S. Pat. No. 5,640,042. That patent shows a planar ferroelectric varactor, including a carrier substrate layer, a high temperature superconducting metallic layer deposited on the substrate for lattice matching, a thin film ferroelectric deposited on the metallic layer, and metallic conductors for connecting the varactor to radio frequency transmission lines. Other tunable dielectric varactors are shown in PCT patent applications PCT/US99/24161 and PCT/US99/26113, and U.S. patent application Ser. No. 09/660,309, which is hereby incorporated by reference. In some varactor applications, is it desirable to provide a tunable varactor that has a relatively low capacitance.
It would be desirable to have a tunable dielectric varactor that does not require a superconducting layer, and can operate at room temperature, has low dielectric losses and can be constructed to obtain relatively low capacitances (typically less than 2 pF).
Voltage tunable dielectric varactors constructed in accordance with this invention include a substrate having a first dielectric constant and having a generally planar surface, first and second electrodes positioned on the generally planar surface of the substrate, the first and second electrodes being separated to form a first gap therebetween; a tunable dielectric layer positioned on the first and second electrodes and in the first gap, the tunable dielectric layer having a second dielectric constant greater than the first dielectric constant; and third and fourth electrodes positioned on a surface of the tunable dielectric layer opposite the first and second electrodes, the third and fourth electrodes being separated to form a second gap therebetween.